$-\dfrac{5}{10} + \dfrac{9}{12} = {?}$
Solution: $ = - {\dfrac{5 \times 6}{10 \times 6}} + {\dfrac{9 \times 5}{12 \times 5}} $ $ = - {\dfrac{30}{60}} + {\dfrac{45}{60}} $ $ = - \dfrac{{30} + {45}}{60} $ $ = \dfrac{15}{60}$